Forces and MotionChapter 2

Pages 36-59http://www.physicsclassroom.com/mmedia/

Gravity and Motion

• Greece 400 BC, Aristotle thought the rate at which an object would fall was based on its mass. He never tested his hypothesis.

Gravity and Falling Objects• Late 1500s, Galileo quested

Aristotle's idea.• Galileo tested two

cannonballs with different masses drop from the Leaning Tower of Pisa. Supposedly , people watching claimed both cannonballs landed at the same time.

• Galileo’s work changed how people understood how gravity of falling objects.

Iron ball

Wood ball

Gravity and Acceleration

• Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects.

WHY???• Acceleration depends on

force and mass.• YES, a heavier object

experiences a greater gravitational force BUT it also is harder to accelerate SO it balances out.

Acceleration due to Gravity

• Acceleration is the rate at which velocity changes over time.

• Falling objects accelerate at a constant rate.

• Objects falling to Earth accelerate at a rate of 9.8 meters per second, or 9.8 m/s/s or 9.8 m/s²

Velocity of Falling Objects

• Calculate the change in velocity (Δv)

• The change in velocity is the difference between the final velocity and the starting velocity.

• Formula: Δv=g x tg is the acceleration due to gravity on Earth (9.8 m/s²)t is the time the object falls in seconds

Examples of Velocity of Falling ObjectsPage 37 examples 1-4

• Example 1: Δv= 9.8 m/s² x 2 sAnswer is 19.6 m/s downward• Example 2: Δv= 9.8 m/s² x 4.5 sAnswer is 44.1 m/s downward• Example 3: 98 m/s ÷ 9.8 m/s² = tAnswer is 10 s• Example 4:14.7 m/s ÷ 9.8 m/s² = tAnswer is 1.5 s(please note that 1 m/s is approx. 2 miles per hour)

Air Resistance and Falling Objects

• Air resistance is the force that opposes the motion of objects through air. (this is why a crumpled piece of paper falls faster than a flat sheet of paper because of surface area)

• Air resistance depends on SIZE, SHAPE, and SPEED of an object.

Acceleration Stops at the Terminal Velocity

• Terminal velocity is the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity.

• Terminal velocity is good for slowing down hail and sky diving.

Free Fall Occurs when there is NO Air Resistance

• Free Fall can only when there is NO air. Either space or a vacuum (a place with no matter).

• Free Fall is the motion of a body when only the force of gravity is acting on the body

Orbiting Objects are in Free Fall

• Astronauts float in orbiting spacecrafts because of free fall NOT because they are weightless.

Two Motions Combine to Cause Orbiting

• An object is orbiting when it is traveling around another object in space.

• See example on page 40.

• Spacecrafts and satellites

• The Moon orbits Earth• Earth orbits the Sun

Orbiting and Centripetal Force• Any object in a circular motion is

constantly changing direction. Because an unbalanced force is necessary to change the motion of any object, there must be unbalanced force working on any object in circular motion.

• The unbalanced force that causes the objects to move in a circular path is known as centripetal force. The word centripetal means “toward the center.”

• This is why the Moon stays in orbit around Earth.

Projectile Motion and Gravity• Projectile motion is the

curved path an object follows when it is thrown or propelled near the surface of the Earth. It has two components- horizontal and vertical motion, both independent of each other.

• Examples: frog leaping, water sprayed by a sprinkler, swimmer diving into water, balls being juggled, an arrow shot by an archer

Horizontal Motion• Throwing a ball your hand

exerts a force on the ball that makes the ball move forward. This gives the ball its horizontal motion, which is parallel to the ground.

• The ball’s horizontal velocity remains constant but the ball’s vertical velocity increases because of gravity causing it to accelerate downward …these two motions cause the curved path.

Vertical Motion

• Vertical motion is motion that is perpendicular to the ground.

• Gravity gives vertical motion to an object in projectile motion.

Newton’s Laws of Motion

• 1st Law of Motion: An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line UNLESS acted on by an unbalanced force.

Newton’s 1st Law (a.k.a. the Law of Inertia)

• Objects will remain at rest, not moving UNLESS…

• Objects will continue to move at specific velocity UNLESS…

• Friction is usually the unbalanced force that causes objects to stop

• Law of Inertia: tendency for all objects to resist ANY change (either starting or stopping) motion

Interactive

• http://science.discovery.com/games-and-interactives/newtons-laws-of-motion-interactive.htm

• Play 1st law

NEWTON’S 1st LAW OF MOTION

EXAMPLE # 1

What causes the motion of the car to change?

NEWTON’S 1st LAW OF MOTION

EXAMPLE # 2 – Explain why the test tube breaks.

Newton’s 2nd Law of Motion

• The acceleration of an object depends on the mass of the object and the amount of force applied.

• As mass increases/ acceleration decreases and vice versa

• Acceleration increases as the force increases

• F = m x a

Newton’s 2nd Law

• Try Math Focus on page 481. a = F ÷ m68.8 N ÷ 7 kg = 9.8 m/s²2. F = m x a1250 kg x 40 m/s² = 50000 N3. F = m x a175 kg x 2 m/s² = 350 N

Second Law

• Look at the animation.Why do the elephant andfeather hit the ground atthe same time? (assume no air resistance)

Source: http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/newtlaws/efff.html

Newton’s 3rd Law of Motion

• Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first

• Force pairs do not act on the same object

• All forces act in pairs – action and reaction

• The effect of a reaction can be difficult to see

Third Law

• Look at the animation. This shows an action with an equal but opposite reaction.

• Can you think of any other equal but opposite force pairs?

Source: http://mw.concord.org/modeler1.3/mirror/mechanics/NewtonCradle.html

Momentum

• The momentum of an object depends on the object’s mass and velocity

• p = m x v• p is momentum, m is

mass, v is velocityTry the # 1 on Math Focus

pg 53:p = 6 kg x 10 m/sp = 60 kg·m/s

Law of Conservation of Momentum• That any time objects collide,

the total amount of momentum stays the same.

• This law applies whether the objects stick together or bounce off each other after they collide

• Objects can either stick together OR bounce off each other after a collision

• Conservation of Momentum can be explained with Newton’s 3rd Law- think billiard balls